I didn't get a chance to reply to this yesterday due to it being a busy Tuesday, but I love the discussion question, so I'm going to reply!

The closest I've been to a fandom of one is in my love for Major (baseball anime). Thankfully, I wasn't quite alone in that one. Actually, one of the previous mods here in this community was also a fan and did some Major fic, which was awesome. But there aren't many of us out there, at least that write (and I haven't written any more fic in the fandom recently).